Concepts - Part III

1. Crossover occurs at the four strand stage which is after the Chromosomes have replicated to produce sister chromatids. Crossing over is the physical exchange between chromatids with two homologous chromosomes participating in the event. There is a reciprocal exchange of equal and corresponding segments between them. Crossing over is assumed to be random along the length of the chromosome.
   
   
2. The closer two loci are located along a chromosome, the lower the frequency of crossing over.
   
   
3. In higher plants double fertilization results in 3n endosperm tissue and 2n embryo tissue.
   
   
4. If two loci are linked, the probability of any two locus combination of alleles is not the probability of one allele times the probability of the second allele at the second locus. This is because the two alleles, each at a different locus, do not segregate independently. Parental combinations of alleles are more frequent than non-parental (recombinant) combinations of alleles, unless recombination is 50%.
   
   
5. 100% crossover means that a crossover event occurs on the average, once for each meiotic event. Only two of four strands participate in a single crossover event. The result of a single crossover event is 50% recombinant gametes and 50% parental gametes.
   
   
6. If r is the observed recombination fraction, then 2r is the relative frequency of single crossover events.
   
   
7. The concept of orthogonal comparisons is that the results of one comparison do not influence the probality associated with the test of significance for the other comparisons.
   
   
8. Coupling phase linkage F₂ families provide more information than repulsion F₂ families when the number of individuals is the same for both types of families.
   
   
9. The recombination fraction can be determined directly from testcross progeny because we know the genotype of each individual.
   
   
10. The effect of dominance gene action is that we can identify phenotypes, but we cannot distinguish the genotypes of all individuals in all four classes of the di-hybrid ratio. If we did not have dominance gene action, we would have ten classes of F progeny, rather than four classes and we could identify the genotypes of nine of the ten classes. If we did not have dominance gene action, we would have more information than we have with dominance gene action.
    
    
11. The product ratio is used to determine the recombination fraction for F₂ progeny with complete dominance gene action. We calculate the product ratio and then use a table of values to solve for 'p'.
    
    
12. The information content of a family is the inverse of the variance of p. The greater the precision of p, the greater the information content of the family. The variance of p depends both on the type of linkage phase (coupling of repulsion) and the number of individuals in the family. The smaller the variance of p, the greater the information content.
    
    
13. When we have linkage data from several types of families we can use all the information to estimate a weighted average for p. More weight is given to families with more information content. In other words, the more percise the estimate of p, the more weight associated with that estimate of p when determining the combined estimate of p.